1. Field of the Invention
The present invention relates to a method for operating a receiver unit for receiving audio signals from a remote transmission unit via a wireless audio link, wherein an audio signal output of the receiver unit is connected to an audio signal input of a hearing instrument comprising means located at a user's ear or in the user's ear canal for stimulating the user's hearing according to the audio signals from the receiver unit. The invention also relates to a system for providing hearing assistance to a user, comprising a remote transmission unit, a receiver unit for receiving audio signals from the transmission unit via a wireless audio link, a hearing instrument, means for connecting an audio signal output of the receiver unit to an audio signal input of the hearing instrument, wherein the hearing instrument comprises means located at a user's ear or in the user's ear canal for stimulating the user's hearing according to the audio signals from the receiver unit.
2. Description of Related Art
Usually in such systems the wireless audio link is an FM radio link. The benefit of such systems is that sound captured by a remote microphone at the transmission unit can be presented at a high sound pressure level to the hearing of the user wearing the receiver unit at his ear(s). In particular, the microphone of the hearing instrument can be supplemented or replaced by the remote microphone which produces audio signals which are transmitted wirelessly to the FM receiver and thus to the hearing instrument. In particular, FM systems have been standard equipment for children with hearing loss in educational settings for many years. Their merit lies in the fact that a microphone placed a few inches from the mouth of a person speaking receives speech at a much higher level than one placed several feet away. This increase in speech level corresponds to an increase in signal-to-noise ratio (SNR) due to the direct wireless connection to the listeners amplification system. The resulting improvements of signal level and SNR in the listeners ear are recognized as the primary benefits of FM radio systems, as hearing-impaired individuals are at a significant disadvantage when processing signals with a poor acoustical SNR.
Most FM systems in use today provide two or three different operating modes. The choices are to get the sound from: (1) the hearing instrument microphone alone, (2) the FM microphone alone, or (3) a combination of FM and hearing instrument microphones together.
Usually, most of the time the FM system is used in mode (3), i.e. the FM plus hearing instrument combination (often labeled “FM+M” or “FM+ENV” mode). This operating mode allows the listener to perceive the speaker's voice from the remote microphone with a good SNR while the integrated hearing instrument microphone allows to listener to also hear environmental sounds. This allows the user/listener to hear and monitor his own voice, as well as voices of other people or environmental noise, as long as the loudness balance between the FM signal and the signal coming from the hearing instrument microphone is properly adjusted. The so-called “FM advantage” measures the relative loudness of signals when both the FM signal and the hearing instrument microphone are active at the same time. As defined by the ASHA (American Speech-Language-Hearing Association 2002), FM advantage compares the levels of the FM signal and the local microphone signal when the speaker and the user of an FM system are spaced by a distance of two meters. In this example, the voice of the speaker will travel 30 cm to the input of the FM microphone at a level of approximately 80 dB-SPL, whereas only about 65 dB-SPL will remain of this original signal after traveling the 2 m distance to the microphone in the hearing instrument. The ASHA guidelines recommend that the FM signal should have a level 10 dB higher than the level of the hearing instrument's microphone signal at the output of the user's hearing instrument.
When following the ASHA guidelines (or any similar recommendation), the relative gain, i.e. the ratio of the gain applied to the audio signals produced by the FM microphone and the gain applied to the audio signals produced by the hearing instrument microphone, has to be set to a fixed value in order to achieve e.g. the recommended FM advantage of 10 dB under the above-mentioned specific conditions. Accordingly, heretofore—depending on the type of hearing instrument used—the audio output of the FM receiver has been adjusted in such a way that the desired FM advantage is either fixed or programmable by a professional, so that during use of the system the FM advantage—and hence the gain ratio—is constant in the FM+M mode of the FM receiver.
CA 2422449 A1 relates to an example of such an FM receiver which not only receives audio signals from a remote microphone transmitter but in addition may communicate with remote devices such as a remote control or a programming unit via wireless link for data transmission.
EP 1 638 367 A2 relates to another example of an FM receiver for receiving audio signals from a remote microphone transmitter, wherein the FM receiver upon receipt of a polling signal from the remote microphone transmitter is capable of transmitting status information regarding the FM receiver to the remote microphone transmitter.
A further example of an FM receiver for receiving audio signals from a remote microphone transmitter is known from U.S. Pat. No. 5,734,976, wherein the FM receiver is equipped with a squelch function by which the audio signal in the receiver is muted if there is excessive noise due to a large distance between the transmission unit and the receiver unit exceeding the reach of the FM link.
WO 97/21325 A1 relates to a hearing system comprising a remote unit with a microphone and an FM transmitter and an FM receiver connected to a hearing aid equipped with a microphone. The hearing aid can be operated in three modes, i.e. “hearing aid only”, “FM only” or “FM+M”. In the FM+M mode the maximum loudness of the hearing aid microphone audio signal is reduced by a fixed value between 1 and 10 dB below the maximum loudness of the FM microphone audio signal, for example by 4 dB. Both the FM microphone and the hearing aid microphone may be provided with an automatic gain control (AGC) unit.
WO 02/30153 A1 relates to a hearing system comprising an FM receiver connected to a digital hearing aid, with the FM receiver comprising a digital output interface in order to increase the flexibility in signal treatment compared to the usual audio input parallel to the hearing aid microphone, whereby the signal level can easily be individually adjusted to fit the microphone input and, if needed, different frequency characteristics can be applied.
Depending on the type of hearing instrument, there are generally two alternatives of how the audio output of the receiver unit is connected to the audio input of the hearing instrument: On the one hand, there are hearing instruments having an audio input which is parallel to the microphone of the hearing instrument and hence has a relatively low input impedance. On the other hand, there are hearing instruments having an audio input which is separate from the microphone of the hearing instrument and which has a relatively high input impedance. In the first case, the microphone of the hearing instrument can be muted by setting the output impedance of the receiver unit to a relatively low value (“FM only” mode), while in the “FM+M” mode the output impedance of the receiver unit is set to a relatively high value in order to allow mixing of the audio output signals of the receiver unit and the hearing instrument microphone signals at comparable levels. The appropriate switching of the output impedance of the receiver unit usually is provided by a manually operable switch at the receiver unit.
In the first case, i.e. in the case of a hearing instrument having a low impedance audio input, one practical problem is that the achieved audio signal levels are often not identical in the “FM only” mode and in the “FM+M” mode. This is caused by tolerances of the audio input impedance of the hearing instrument due to variations of the impedance of the microphone of the hearing instrument and by the fact that the audio output impedance of the receiver unit is fixed and also has tolerances. Practically, a spread of the hearing instrument input impedance as large as from 2 kOhm to 11 kOhm has been measured. Usually the desired FM advantage, which theoretically could be predetermined by setting the gain applied to the audio signals in the receiver unit and/or the audio output impedance of the receiver unit accordingly, in practice is achieved only for a hearing instrument having a microphone which has exactly the impedance value (e.g. 3.9 kOhm) assumed when setting the gain and/or audio output impedance. In other words, in practice the desired FM advantage usually will not be achieved due to the practical variations of the audio input impedance of the hearing instrument.
In the second case, i.e. in the case of a hearing instrument having a high impedance audio input, switching between the “FM only” mode and the “FM+M” mode is done within the hearing instrument. In this case, the output impedance of the receiver unit should be set to the low value in order to achieve the desired FM-advantage. If the receiver unit is used at the high output impedance setting, the desired FM-advantage will not be achieved.
A further problem occurring with FM systems results from the fact that the receiver unit has to be mechanically and electrically connected to the hearing instrument, usually via a so-called “audio shoe”. It may happen that there is no electrical connection between the audio output of the receiver unit and the audio input of the hearing instrument. In this case the wireless audio link will not be working, which, however, may not be recognized by the user, in particular if the user is a child.
It is an object of the invention to provide for a method for operating a receiver unit for receiving audio signals from a remote transmission unit via a wireless audio link, which receiver unit is connected to an audio signal input of a hearing instrument, wherein variations of the actually provided audio signal level due to variations of the input impedance of the hearing instrument should be reduced. It is a further object to provide for such a receiver unit.
These objects are achieved by a method as defined in claim 1 and a receiver unit as defined in claim 28, respectively.